Digital electronics, Electronics

Digital Electronics: Combinational circuits

In this post, I will explain how to design combinational circuits, which are just digital circuits with combinations of many logic doors.

Logic gates

We have the following logic doors in IEEE standard.

The exclusive OR (XOR) is a combination of logic gates as shown in the figure below and exclusive NOR (XNOR) is XOR with an inverter.

Physically, you can build logic gates with BJT transistors, diodes or CMOS. The CMOS mode is used for integrated circuits.

How to design combinational circuits?

There are the steps to design and build a digital combinational circuit, with an example.

Step 1: Create the truth table.

Step 2: Write in AND to all cases which have “1” output and create an expression in this form.

Step 3: Simplify the expression, use the Karnaugh map, I will explain in other post.

Step 4: Implement the circuit.

Commercial chips

You can use commercial chips to implement combinational circuits in practice. Here are some few examples.

74LS04

                                       

74LS02

74LS00

74LS08

74LS32

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About Pedro Ney Stroski

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